How to solve "Cannot resolve symbol R" error in Android studio?

Question

android studio

Lately, "Cannot resolve symbol R" error has started popping up in Android studio.
Help fix.
Ad Mob did not add, cleaning the project does not help.
The code in which the error appears:

package com.helloworldstudio.a4;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public void onClick(View view) {
        Intent intent = new Intent(MainActivity.this, activity_second.class);
        startActivity(intent);
    }

    public void onTap(View view) {
        Intent intent = new Intent(MainActivity.this, activity_third.class);
        startActivity(intent);
    }
    public void onContact(View view) {
         EditText userEditText = (EditText) findViewById(R.id.editTextUser);
         EditText giftEditText = (EditText) findViewById(R.id.editTextGift);

                Intent intent = new Intent(MainActivity.this, activity_second.class );

        intent.putExtra("username", userEditText.getText().toString());
        intent.putExtra("gift", giftEditText.getText().toString());
        startActivity(intent);

    }


    @SuppressLint("Registered")
    class activity_second extends Activity {

        @SuppressLint("SetTextI18n")
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_second);
            String user = "Ничего";
            String gift = "ничего";

            user = getIntent().getExtras().getString("Username");
            gift = getIntent().getExtras().getString("gift");

            TextView infoTextView = (TextView)findViewById(R.id.textViewInfo);
            infoTextView.setText(user + " , вам передали " + gift);

        }
    }

Answer 1

Import class R

Answer 2

Or a resource problem. For example, somewhere not set a tag in xml.

Answer 3

class R is missing in the import section of the specified code. Your CO